CHANGES IN THE CIRCULATION. 307 



method. The horizontal distances, since they are determined hy the 

 rate at which the paper moves, correspond to the time. For example, 

 if the paper moves an inch per minute, then, if we measure up a 

 horizontal line the number of inches along which the tracing continues 

 to rise, we can fix the points corresponding to a given time, and mea- 

 sure the height to which the pen had risen at any given instant, and 

 also the rate at which it rose. 



The vertical motion of the pen depends on the rise and fall of the 

 test-tube M, which in its turn is caused by the out- and in-flow of the 

 water surrounding the arm. Hence, a rise in the curve corresponds to 

 an expansion, a fall to a contraction of the forearm. There are other 

 details and precautions in the construction and use of the apparatus 

 requisite to secure entire accuracy, for which I must refer to the origi- 

 nal memoir. Modifications of the apparatus have been made by 

 Ludwig and Kronecker, and a very essential improvement has been 

 suggested and used by Dr. H. P. Bowditch, of Boston. These im- 

 provements are important to the experimenter, but have only a sub- 

 sidiary interest for the general reader. The plethy sinograph can be 

 employed for many purposes. For example, it may serve as a very 

 perfect thermometer, by applying it to record the changes of a closed 

 volume of air, which of course increases or diminishes according as the 

 temperature rises or falls. 



With the aid of the plethysmograph Mosso discovered that the 

 activity of the brain is directly connected with the circulation of the 

 forearm. The importance of this fact will be better understood, if it 

 is considered that we here have a physical phenomenon we can accu- 

 rately measure, directly connected with mental phenomena we can not 

 measure. 



It is well known that the functional activity of an organ of the hu- 

 man body is accompanied by increased activity of the circulation, and 

 this is also true of the brain. Since the total amount of blood is not 

 subject to rapid and sudden changes, it is evident that, when an active 

 part receives more blood, other parts must receive less. When the 

 brain is at work, blood is withdrawn from the arm, which therefore 

 becomes smaller. 



In order that the volume of the forearm should remain constant, 

 absolute tranquillity is necessary ; the slightest movement of the mind 

 suffices to disturb the equilibrium of the vascular system. The ple- 

 thysmograph is an instrument with which we can record even emotions 

 not depicted by any expression of the countenance, or revealed only 

 by unnoticeable changes in the beating of the heart or of the respira- 

 tory movements. " Therefore," says Mosso, " I think it my duty to 

 commence with a tracing that represents the sentiment of veneration 

 which I felt in the presence of my beloved master. Behold, in fact, 

 the contraction of the vessels produced by the entrance of Professor 

 Ludwig, every time he honored the researches made upon myself with 



